This invention relates to optical waveguide fibers and, in particular, to tight buffered optical waveguide fibers having improved strippability.
As is well known in the art, optical waveguide fibers generally comprise a core and a cladding, wherein the core has an index of refraction which is greater than that of the cladding. At the time of manufacture, the cladding is normally coated with one or more layers of a thin plastic material such as a UV-curable acrylate polymer. As used herein, this initial protective layer or layers will be referred to collectively as the fiber's "first protective coating." Typical outside diameters (OD's) for these components are approximately 10 microns for a single mode core (or 50-62.5 microns for a multimode core), 125 microns for the cladding, and 250 microns for the first protective coating.
Because of their relatively fragile nature and because they suffer high increases in attenuation when subjected to tensile, bending or torsion strains, optical waveguide fibers are normally surrounded with at least one relatively thick protective layer which serves to "buffer" the fiber from its surroundings. The protective layer can be spaced from the fiber to form a "loose tube" construction, or can be in direct contact with the fiber to form a "tight buffered" construction. The present invention is concerned with tight buffered fibers.
Numerous tight buffered fibers have been disclosed in the art. See, for example, Yamamoto et al., U.S. Pat. No. 3,980,390, Fuse et al., U.S. Pat. No. 4,629,286, and Stiles, U.S. Pat. No. 4,365,865. Typically, the protective (buffer) layer (also referred to herein as the "second" coating) has a thickness of around 325 microns, so that the overall construction has an outside diameter of around 900 microns. Various materials have been used for the buffer layer including nylons, polyesters, and polyvinyl chlorides.
A recurring problem with this type of construction involves removing (stripping) all of the protective layers from the fiber so as to expose the cladding, i.e., stripping both the buffer layer and the first protective coating from the cladding. Hand operated and motorized tools have been developed for this purpose. See, for example, Zdzislaw, U.S. Pat. No. 4,748,871, and Lukas, U.S. Pat. No. 4,852,244.
Also, first protective coatings having reduced adhesion for the glass cladding have been proposed. See Ansel et al., U.S. Pat. No. 4,472,021, which discloses a coating comprising a UV-curable acrylate compound and an organic polysiloxane, Kondow et al., U.S. Pat. No. 4,660,927, which discloses a silicone coating which can be cured without heating which is said to reduce coating/cladding adhesion, and Suzuki, U.S. Pat. No. 4,642,265, which discloses a silicone coating which includes between 1 and 50 percent by weight of an amorphous silica powder having an average particle size of less than 0.2 microns. Nevertheless, problems have remained because of the tight adhesion between the buffer layer and the first protective coating.
Marx et al. U.S. Pat. No. 5,011,260, filed July 26, 1989, seeks to address this problem by incorporating an ultra-thin layer (0.3 to 0.5 microns) of a decoupling material between the first protective coating and the buffer layer. The decoupling material disclosed is a copolymer blend of polyacrylates sold by the Monsanto Company under the trademark MODAFLOW. The material is water insoluble and thus must be blended with an organic solvent, such as acetone, for processing. The reference describes the use of 97.5% solvent by weight in the blend. Removal and proper disposal of this quantity of a highly volatile and flammable material is a clear disadvantage of this approach.
The incorporation of a release agent between the first protective coating and the buffer layer of an optical waveguide fiber has been disclosed in a number of other references. In particular, Claypoole et al., U.S. Pat. No. 4,072,400, discloses the use of silicone oil, a petroleum lubricant, a layer of colloidal graphite, or talc for this purpose.
The use of silicone oil at the interface between the first protective coating and the buffer layer is also disclosed in Japanese Patent Publication No. 62-99711 and in an article entitled "Low-Temperature Excess Loss of UV-Curable Acrylate/Nylon-Coated Optical Fibres" by H. Itoh, T. Kimura, and S. Yamakawa, Electronics Letters, Vol. 20, No. 21, Oct. 11, 1984, pages 879-881. These references are concerned with the problem of excess signal loss at low temperatures for fibers having a first protective coating composed of a UV-curable polymer and a buffer layer composed of nylon.
In addition to the use of a layer of silicone oil 2-3 microns thick, the Itoh et al. article also discloses using a 50 micron layer of silicone rubber between a first protective coating made of a polybutadiene acrylate and a buffer layer made of nylon. Similarly, the Japanese patent publication states that in addition to silicon system resins covering silicone oils, the powders of fluorine system resins can also be used as a mold releasing agent. The Japanese publication does not include any examples using the powders of fluorine system resins, nor does is disclose any suitable resins or how such resins are to be applied to the first protective coating. Moreover, there is absolutely no disclosure in the reference of the use of a binder in connection with an interfacial layer.
Optical waveguide fibers employing TEFLON polymers as part of a protective system have been disclosed. Thus, Suzuki, U.S. Pat. No. 4,741,594, discloses the use of expanded, porous polytetrafluoroethylene (PTFE) as a buffer material for optical waveguide fibers. Johnson et al., U.S. Pat. No. 4,723,831, and Gartside, III et al., U.S. Pat. No. 4,826,278, disclose loose tube constructions employing core wraps composed of woven fiber glass impregnated with PTFE (Johnson) and TEFLON tape (Gartside). A loose tube construction employing a fluoropolymer tube is disclosed in The Fiber Optic Catalog--1988-1989, Siecor Corporation, Hickory, N.C., page 1.20, 1988.